Microdisplays generally produce magnified virtual images of patterns formed by microdisplay engines, which typically function as spatial light modulators for patterning light from external sources or as self-illuminators that produce patterns of light from arrays of controllable internal sources. Technologies employed for spatial light modulation include liquid crystal diode (LCD), liquid crystal on silicon (LCoS), or digital light processing (DLP). Self-illuminating display engines can be based on organic light-emitting diode technology (OLED).
Focusing systems of the microdisplays magnify the patterns formed by the microdisplay engines as virtual images that are visible within limited regions of space referred to as “eyeboxes”. In binocular microdisplay systems, separate eyeboxes are provided for each eye. Eyeboxes larger than the typical viewer's eye pupil size can waste light but can accommodate relative displacements between the viewer's eyes and the microdisplays as well as variations in viewer's interpupillary distances for binocular systems.
The microdisplay engines and associated optics of wearable near-eye microdisplays are generally positioned out of the viewer's line of sight for both functional and aesthetic reasons. Non-immersive microdisplays preserve line-of-sight views of the ambient environment. Beamsplitters along the line-of sight admit light from the ambient environment while folding the light paths from the microdisplay engines into alignment with the line of sight. The folded light paths of the microdisplays provide for designs that are more compact. For example, the required focal lengths of the powered optics are generally achieved by locating the microdisplay engines at a distance above or to the side of the viewer's eyes along folded light paths.
The transverse height and width of the light beam at the exit pupil of the microdisplay required for filling the eyebox influence the size of the optics in corresponding dimensions. In a folded system, where a beamsplitter redirects light from the microdisplay engine to the viewer's eye from one side of the viewer's eye, the thickness of the microdisplay in front of the eye is influenced by the required beam width. Prior to folding, the dimension of the beamsplitter along the line of sight is generally sized to accommodate the beam width.
A longstanding goal of wearable near-eye microdisplays has been to limit the thickness of the microdisplays in front a viewer's eyes so as to resemble conventional eyeglasses more closely. Thinner displays are also desirable for other purposes, including for purposes of integration with other devices such as hand-held electronic devices.
Attempts have been made to provide substrate-guided beam expanders in front of viewers' eyes to convey image-bearing light beams toward viewers' eyes from off-axis positions by total internal reflection and to reorient the light beams in alignment with the viewers' lines of sight through a series of partial reflections that effectively expand the light beams in the width direction. Each in the series of partially reflective interfaces within a guided substrate redirects a portion of a light beam's energy in a direction generally toward the viewer's eye. The partial reflections disperse the light beam's energy in the width direction to fill the desired eyeboxes.
Such beam expanders containing internal partially reflective interfaces are difficult to manufacture and are subject to problems such as ghost imaging or angular sensitivities at interfaces that complicate requirements for directing light rays in desired directions.